Understanding the structure and properties of geological formations can reduce the cost of drilling wells for oil and gas exploration.
Measurements made in a borehole (i.e., downhole measurements) are typically performed to attain this understanding, to identify the composition and distribution of material that surrounds the measurement device downhole.
In wireline logging, the measuring devices attached to a sonde are lowered and retrieved from the borehole by an armored cable that provides electrical power and a pathway for transferring acquired data to the surface. Alternatively, in logging-while-drilling (LWD), the measurement sensors are placed close to the bit, providing information to drillers and geologists at the surface via telemetry.
Many sensors in wireline logging and LWD operate using electromagnetic (EM) principles. In some reservoirs, the desired detail in formation evaluation is provided with borehole imaging tools. For example, in so-called dipmeter tools, small electrodes are mounted on several pads placed at various circumferential positions to provide image data from around the borehole. Dipmeter processing is employed for a quick determination of structural dip, and for locating and orienting structural events. With such information, a more comprehensive seismic interpretation is subsequently performed. However, due to the size limitations of EM sensors and electrodes, full circumferential coverage of the borehole using a dipmeter tool is usually not practical.